The present invention relates generally to a technique for converting images in scale or resolution, and more specifically to a technique in which binary input image data of one resolution is converted to gray output image data of another resolution.
Image information, be it color or black and white, is commonly generated in a bitmap format at a particular scale, orientation .THETA. and resolution K.times.L.times.b, corresponding to a desired printer output, where K is a number of spots per unit of length in one dimension, L is a number of spots per unit length in the other dimension, and b is the depth of each pixel, in number of levels. This bitmap is present for every color separation of the output device, i.e., 4 bitmaps for a 4-color output device, 3 for a 3-color, 2 for a 2-color and 1 for a black and white output device. In a common example of a black and white output, image data comprising a bitmap to be printed is provided to a printer suitable for printing at 300 spots per inch (spi) in both dimensions, at a one bit depth giving 2 levels. Many considerations drive this single selection of resolution, including the desirability of providing only a limited number of fonts (alphanumeric bitmaps) so as to use only a limited amount of storage space. Common software packages available on personal computers or for operation of input scanners for document creation also usually provide only a single resolution output. Images are commonly arranged at a fixed orientation
Increasingly, the resolution available from printers varies over a wider range of choices. Printer resolutions are available over a range, for example, from less than 200 spi to more than 600 spi. Resolutions vary for a number of reasons, generally related to the quality of the output image. Simply printing a 300 spi bitmap at 400 spi or 600 spi is undesirable however, since the image will be reduced substantially in size on the output page or display. It would be highly desirable to provide the capability of printing any image at any resolution, while selecting the output size and orientation.
It is a common practice to implement conversion of a bitmap of first resolution K.times.L.times.b to a bitmap of second resolution M.times.N.times.d through simple pixel level and raster level operations, such as pixel doubling. When resolution is changed by integer multiples, the resolution conversion can be achieved by pixel replication. Pixel replication, however, does not best use the capabilities of higher resolution output devices. Hence in performing resolution conversion with a higher resolution output device it is desirable to perform a smoothing operation.
Scaling is an identical operation to resolution conversion, and the number of pixels representing the image is either increased or decreased. In particular, a conversion is implemented for a bitmap of first resolution K.times.L.times.b to a bitmap of second resolution M.times.N.times.d through simple pixel level and raster level operations, such as pixel doupling, but if the image is then directed to a K.times.L.times.b for output, it is considered scaled.
Alternatively, more elaborate methods than pixel replication have been developed to facilitate resolution conversion, some of which are described in the following patent applications:
U.S. patent application Ser. No. 07/513,415; Filed: Apr. 23, 1990; Title: "Bit-Map Image Resolution Converter" PA0 U.S. patent application Ser. No. 07/588,125; Filed: Sep. 25, 1990; Title: "Bit-Map Image Resolution Converter Compensating for Write-White Xerographic Laser Printing" PA0 U.S. Pat. No. 4,630,947; Patentee: Yoshida et al. Issued: Dec. 23, 1986. PA0 U.S. Pat. No. 4,829,587; Patentee: Glazer et al. Issued: May 9, 1989. PA0 U.S. Pat. No. 4,907,152; Patentee: Lempriere; Issued: Mar. 6, 1990
Each of the above-mentioned patent applications contemplates a method of magnifying, by a predetermined magnification factor (n), original image pixels in two dimensions. The method includes the steps of selecting an original image pixel, as well as determining the binary state of both the selected original image pixel and all immediately surrounding original image pixels. Additionally, the selected original image pixel is expanded into an n.times.n array of magnified image pixels to represent the magnification of the selected original image pixel. Finally, a binary state is assigned to each pixel in the array of magnified image pixels according to the pattern of binary signals previously determined for the selected original image pixel and all immediately surrounding original image pixels. In these patent applications, the assignment of the binary states to the pixels in the array of magnified image pixels is made according to a set of state determination rules.
Employing sophisticated resolution conversion techniques does not insure that the resulting output image will have a desirable appearance. For instance, the output image can be excessively blocky and/or contain noticeable "jaggies." Hence, smoothing operations are sometimes used in conjunction with the conversion or scaling of the image. Through use of the state determination rules in the techniques of the above-mentioned applications, smoothing operations are achieved. For example, the method of the patent application entitled "Bit-Map Image Resolution Converter" permits such operations as smoothing of edges, smoothing of half-bitting effects and smoothing of lines. Moreover, the method of the patent application entitled "Bit-Map Image Resolution Converter Compensating for Write-White Xerographic Laser Printing" ("Compensating for Write-White Application") permits enhancing of single bits in printing and compensating for loss of resolution in write-white printing. Indeed, the compensating technique disclosed in the Compensating for Write-White Application is presently employed in the DocuTech.RTM. electronic printer manufactured by Xerox.RTM. Corporation. The pertinent portions of the above-mentioned patent applications entitled "Bit-Map Image Resolution Converter" and "Bit-Map Image Resolution Converter Compensating for Write-White Xerographic Laser Printing" are incorporated herein by reference.
The following patents relate to the area of resolution conversion:
U.S. Pat. No. 4,630,947 discloses an enlarged pattern generator for generating an enlarged dot matrix pattern from an original dot matrix pattern by providing up to four mutually adjacent dots.
U.S. Pat. No. 4,829,587 discloses a device for rescaling an image in which a gray level output is produced by convolving the image with a filter kernal.
U.S. Pat. No. 4,907,152 discloses a method for increasing the resolution of a computed tomography (CT) scanner where the density of the captured data is averaged by use of density values obtained on successive first and second scans, wherein the first scan is offset from the second scan.
While an advantageous technique for using state determination rules to convert input image data of one resolution to output image data of a second resolution is disclosed by U.S. patent application Ser. No. 07/513,415, these rules are only be used to convert binary input image data to binary output image data. It would be desirable to provide a rules-based technique in which binary input image data of one resolution is converted to gray output image data of another resolution.